The invention relates to a radio guidance antenna, a data communication method, and a non-contact data communication apparatus, which make use of such antenna, and more particularly, to a radio guidance antenna for use in non-contact identification apparatus such as physical distribution systems, electronic coupon ticket systems, and the like, a data communication method, and a non-contact data communication apparatus, which make use of such antenna.
Conventionally, a system for identification and management of articles is needed in article identification apparatus such as assembly and conveyance lines and physical distribution systems, and electronic coupon ticket systems.
FIG. 21 is a view showing the schematic constitution in such system. As shown in FIG. 21, data carriers (referred below to as tags) 201, 202 of a non-contact identification apparatus are fabricated in a card-shape and a coin-shape to contain therein printed coils 203, 204 and IC chips 205, 206. These tags 201, 202 are attached to commodities 207 to be managed, and data are transmitted and received in a non-contact manner as the commodities are passed through antenna gates 208, 209. Thus, the tags are used as a tool of merchandise management and conveyance history management in the field of physical distribution, security and so on.
Radio guidance antennas are housed in the antenna gates 208, 209 of the non-contact identification apparatus shown in FIG. 21. The most important point required for such radio guidance antennas is to ensure the magnetic-field intensity necessary for communication in all locations in a read area. Communication between a read and write device of the non-contact identification apparatus and the tags 201, 202 makes use of mutual inductance coupling between antennas for transmission and reception and loop antennas 203, 204 formed in the tags 201, 202.
Induced electromotive forces generated in the loop antennas 203, 204 of the tags 201, 202 can be represented by—M (di/dt) where M indicates mutual inductance between the antennas for transmission and reception and the loop antennas 203, 204 in the tags 201, 202 and i indicates electric current generated in the antennas for transmission. This means that in order to ensure a predetermined magnetic-field intensity when i=constant, mutual inductance M of at least a predetermined value must be generated. That is, in the case of M=0, electric power is not supplied to the tags 201, 202 however great the current through the read antennas may be, and so communication between the read and write antennas and the tags 201, 202 becomes impossible.
With conventional antennas, which are in many cases disposed on a single plane, however, regions where M=0 or M is very small are always present in read and write regions.
FIG. 22 shows mutual inductance between loop antennas of one winding. In FIG. 22, lines of magnetic flux emitted from a transmission antenna 220 are indicated by solid lines with arrows, and it is shown that the more lines of magnetic flux per unit area, the larger magnetic flux density. Also, the magnetic flux density, at which magnetic flux generated by current through the transmission antenna 220 passes through an antenna loop of a tag, is in proportion to M between the read and write antenna and an antenna of the tag. Accordingly, it is shown that the more the number of lines of magnetic flux passing through the loop of the tag, the larger the mutual inductance M.
A tag 211 shown in FIG. 22 is disposed on the same axis as that of the transmission antenna 220, so that a transmission antenna loop and a loop of the tag are in parallel to each other. In the case of such positional relationship, it is shown that the number of interlinkages of lines of magnetic flux generated by the transmission antenna 220 is large and the mutual inductance M is large. In contrast, in the case where a tag 212 is disposed so that a loop of the transmission antenna 220 and a loop of the tag are perpendicular to each other, the lines of interlinking magnetic flux become 0, that is M=0.
FIG. 22 also shows a tag 213 which is parallel to the transmission antenna 220 but disposed in a position offset from a surface of projection of the transmission antenna 220 in an axial direction. In this case, the number of lines of magnetic flux making interlinkage with the tag 213 is very small and the mutual inductance M becomes small. In the case of an antenna system with the transmission antenna 220 and only one feeding point, a region or regions where the mutual inductance M is 0 or very small are always present depending upon the position and direction of a tag. Accordingly, when such arrangement is used in an antenna system, in which a tag is not limited in orientation and a predetermined mutual inductance M is generated in a large area, it has been naturally necessary to increase the number of antennas and feeding points.
FIG. 23 shows mutual inductance between loop antennas when there are provided two transmission antennas. Like the case in FIG. 22, a magnetic field radiated from a transmission antenna 221 provided in addition to the transmission antenna 220 is represented by lines of magnetic flux indicated by broken lines with arrows. In the case where the two transmission antennas 220, 221 are installed, lines of magnetic flux generated by the transmission antenna 221 pass through tags 212, 213. However, the mutual inductance M between tags 212, 213 and the transmission antenna 220 is not adequate. Thus, the mutual inductance M is generated between the tags and the transmission antenna 221. Accordingly, the more the number of antennas, the more complex the magnetic field, so that there is an increased probability that communication will be enabled irrespective of directions and positions of tags.
However, the above-mentioned measure involves a significant problem. As shown in FIG. 23, many lines of magnetic flux make interlinkage with the transmission antennas 220, 221 and thus the mutual inductance M between the transmission antennas is shown as being increased. That is, a part of electric power supplied to the transmission antenna 220 is also supplied to the transmission antenna 221 due to mutual induction, so that all of the electric power supplied to the transmission antenna 220 is not supplied as an antenna current to the transmission antenna 220. Instead, a part of the electric power supplied to transmission antenna 220 disadvantageously increases the remote electromagnetic-field intensity from the transmission antenna 221.
In this manner, it is very difficult to arrange a plurality of antennas in an overlapping manner and control them independently. Because of this, in the case of using a plurality of antennas, the antennas are conventionally arranged with particular distances therebetween so that mutual inductance between the antennas becomes small, but it becomes difficult to assure the stability of read and write regions.
One way to solve the above-described problem is with a three-dimensionally perpendicular arrangement of antennas as described in Japanese Laid-Open Patent Application No. 2000-251030. However, antennas of such construction have been too complex and expensive to be practical.